1. Field of the Invention
The present invention relates to a lighting device and a display equipment which uses a low-pressure mercury vapor discharge lamp.
2. Description of Related Art
Using a low-pressure mercury vapor discharge lamp as a lighting device and in display equipment is known. Generally, in such lamps, the mercury vapor pressure becomes low and luminescence efficiency will decrease under low temperature conditions. In fact, luminosity can fall to 10% or less as compared to operation at normal temperatures.
To overcome this problem, it has been known to attach a heater to the discharge lamp in order to heat the lamp before it is energized. However, the cost of the heater is high and causes the equipment to become large.
As an alternative solution, a lamp which prevents luminosity from falling at low temperatures, is disclosed in Japanese Utility Model Patent Publication 4-52932. This lamp uses a dual tube arrangement, having an outer bulb and an inner bulb. The inner bulb defines a discharge space therein. Since the outer bulb insulates the inner bulb, luminescence efficiency does not fall as much as without the outer bulb in low-temperature environments. However, the extent to which the outer bulb can maintain the luminescence efficiency in low-temperature environments is limited. Luminescence efficiency is lower than the lamp which does not include the outer bulb, but instead has a heater. Moreover, even when a heater is attached to the outer bulb in a dual bulb arrangement, the heat of the heater is not transmitted to within the inner bulb.
Japanese Patent Laid-open No.7-272888 shows a lighting device which raises the lamp voltage when the temperature around the bulb of a single tube, cold cathode fluorescent lamp is low. However, because this technology employs a single tube, heat dissipation from the bulb is large. Therefore, even when the lamp voltage goes up, the temperature of the lamp does not rise effectively.